This invention is directed to noise suppression and, more particularly, to the suppression of noise created by a jet or fan engine exhaust.
Ever since the advent of jet powered aviation, proposals directed to diminishing the noise produced by jet engines have been made. Many of these proposals have been reduced to practical structures and incorporated into modern jet aircraft. Engines with low and high bypass ratios fall in this category. Initially, low bypass ratios were found adequate to provide the necessary noise suppression. More recently, jet engines with high bypass ratios have been developed to further reduce the noise of the jet exhaust. Other apparatus, such as acoustic nacelles, mechanisms for controlling fan blade tip speeds, etc., have also been developed in an attempt to reduce jet engine exhaust noise. While each of these different approaches has been somewhat successful, noise remains a problem.
It has been noted through jet engine noise research that an array of apertures, for example an array of circular or oval tubes, located external to the engine, effectively move the center of the noise source created by jet and fan jet engines closer to the rear of the engine. In the past, however, this effect has not found widespread use because, even though it provides some noise suppression, it also creates a number of detrimental effects. More specifically, the inclusion of a multiple tube array in a jet exhaust stream has been found to decrease engine thrust and increase drag. Because engine thrust is decreased, fuel economy has also decreased. In other words, engine efficiency is decreased.
It has been suggested by the prior art that multiple tube arrays only be mounted in the jet exhaust stream when noise suppression is desired, i.e., when the aircraft is on the ground or at a low altitude. In this regard, attention is directed to U.S. Pat. No. 3,067,968 issued to Charles W. Heppenstall for "Retractable Jet Engine Noise Suppressor." The major problem with devices of the type disclosed in this patent is that while they provide a mechanical system for withdrawing the multiple tube array during cruise conditions, they do so in a manner that unduly disturbs the wind airflow. Thus, additional problems result from the use of systems of the type disclosed in this patent. Further, because of the engine location, such arrangements cannot take advantage of naturally occurring sound barriers, such as the wings of the aircraft.
In the past few years, development work directed to mounting jet engines above and forward of the wings, as opposed to in (or beneath) the wings, has been undertaken both for subsonic and for supersonic aircraft. In both cases, engine noise suppression is desired. This invention is directed to noise suppression systems suitable for use with such aircraft.
More specifically, as will be better understood from the following description, this invention is directed to a noise suppression system suitable for use in combination with subsonic or supersonic aircraft wherein the jet exhaust path of a jet or fan engine passes chordwise above the wing. The invention is suitable for suppressing noise when suppression is particularly desirable (e.g., during takeoff and landing, and when the aircraft is on the ground).
It is an object of this invention to provide a jet or fan engine noise suppression system.
It is a further object of this invention to provide a jet or fan engine noise supression method and apparatus particularly suitable for use with aircraft having engine exhaust streams that pass above the wings of the aircraft.